The ability to be more specific with QRS axis deviation requires a quick scan of all the limb leads (I, II, III, aVR, aVL, aVF). Using these standard and augmented voltage leadsThese lead views are mathematically created from the limb electrodes to view the heart from three distinct positions towards the centre of the heart. Therefore, the augmented voltage right (aVR) views the right atria and ventricle from the right shoulder..., a closer approximation can be made of the QRS axis allowing for the identification of shifts in the QRS axis within each quadrant.
A more specific QRS axis is established with a simple two step process. The six leads created by the four limb electrodesThe 12-lead ECG provides 12 views of the heart. The 12 views are generated through only 10 electrodes. Four limb electrodes attach to the inner forearms and calves. The remaining 6 electrodes, labelled successively from V1 to V6, are attached... are ideal when calculating the QRS axis since these leads (aVR, aVL, aVF, I, II, III) provide six directions within the frontal plane. The six surface leads provide a full 360 degree reference system, separated by 30 degree intervals (see Figure 6.16).
By studying the amplitudeThe height or depth of waves and complexes of an ECG in millimetres; represents millivolts where 10 mm is 1 millivolt with a properly calibrated monitor. 1. Six Second ECG Guidebook (2012), T Barill, p. 190 and direction of the QRS complexThe electrical representation of ventricular depolarization; the atrial repolarization is also a part of the QRS. ECG interpretation relies heavily on the QRS complex. The QRS complex represents the depolarization of the ventricles. The repolarization of the atria is also... in each of these six leads, the QRS complexA collection of waveforms (i.e. QRS complex and the ECG complex). 1. Six Second ECG Guidebook (2012), T Barill, p. 194 with the greatest amplitude (upright or downward facing) has a QRS axis that is near parallel the lead chosen. The lead with the QRS complex that is diphasic (positive deflection is equal the negative deflection) is perpendicular the QRS axis.
Of the 6 frontal leads (I, II, III, aVR, aVL, and aVF), the lead with the largest QRS (height up or down) is approximately parallel to the electrical axisThe direction of an average (mean) vector is called the electrical axis. The overall direction of ventricular depolarization is called the QRS axis. Many novice practitioners of 12 lead ECG interpretation find identifying the QRS axis challenging and somewhat confusing.... of the ventriclesThe larger chambers of the heart (3 times the volume and muscle thickness than the atria), responsible for the pumping of blood to the lungs and the rest of the body.. The lead that has diphasic QRS complexes with equal amplitudes facing up and down is perpendicular to the electrical axis. For example, if lead II has the largest QRS, expect lead aVL (which is perpendicular to lead II) to have biphasic QRS complexes of equal amplitudes up and down.
For example, if after scanning the six leads in question, lead II is found to have the largest amplitude, the QRS axis of the ventricles is parallel to lead II. If the QRS complex in lead II is predominantly upright, then the QRS axis is also moving in the direction of the positive electrode in lead II – in this case, the red electrode. These findings would support a QRS axis that is +60 degrees, a normal QRS axis.
Identifying a QRS axis is a rather simple exercise. Making sense of your results requires the ability to place all clinical information in context. How does this electrical axis compare with previous ECGs? What is the patient’s cardiac history? Having the skill to identify a QRS axis can help build a more complete clinical picture.
(See Electrical Axis DeviationRight Axis Deviation - (more than +90 degrees): pulmonary hypertension, right ventricular hypertrophy, right bundle branch block, normal for children, high lateral MI Left Axis Deviation - (more negative than -30 degrees*): left ventricular hypertrophy, inferior MI, left bundle branch... for more info)
Figure 6.16 provides the 360° reference schematic created by the six limb leads. Upon first glance, this is somewhat overwhelming. With practice, this diagram becomes a simple tool. Begin with leads I and aVF. The coordinates for these leads are already well established with lead I the starting point of 0° and aVF perpendicular (90°)to lead I. The standard limb leads (I,II,III) form Einthoven’s triangleDr. Einthoven was a physiologist, who in the late 1800s, first established the 3-lead cardiac monitoring system; the three lead system maps the electrical workings of the heart with a triangular lead system - Leads I, II, III. The three... with 60° separating each lead. Hence lead II is 60° from lead I and lead III is a further 60° from lead II (120° from lead I). The augmented voltage leads also form the frontal plane with these three leads (aVR, aVL, aVF) covering a full 360°. Each of these leads then are separated by 120°. Lead aVL occupies a QRS axis of -30°. The opposite end of lead aVR provides the +60° point.
1. Six Second ECG GuidebookA Practice Guide to Basic and 12 Lead ECG Interpretation, written by Tracy Barill, 2012 Introduction The ability to correctly interpret an electrocardiogram (ECG), be it a simple six second strip or a 12 lead ECG, is a vital skill... (2012), T Barill, p. 169